The effect of screening of the magnetotelluric field by high resistivity layers: Three-dimensional numerical modeling constraints     

A. N. Kuznetsov  I. V. Egorov 《Izvestiya Physics of the Solid Earth》2006,42(2):161-166

A 3-D numerical modeling approach is applied to the effect of screening of the magnetotelluric (MT) field by high resistance (≥10⁶ Ω m²) layers, which up to now has mainly been studied in terms of 2-D heterogeneous models of geological sections [Berdichevsky and Yakovlev, 1990]. Three-layer models above a poorly conducting basement (ρ=10⁴ Ω m) are used. The resistance of the screening layer was generally taken equal to 2×10⁶ Ω m² and its thickness was varied from 200 to 2000 m. The resistivity of the host medium was set at 10 Ω m. Heterogeneities of a low and a high resistivity (ρ=10^?3 and 10⁴ Ω m, respectively) ranging in horizontal size from 4 to 40 km and having a height of 1–2.8 km (a protrusion on the basement) were examined. Based on calculations of these models, 2-D and 3-D screening effects were compared. The 3-D modeling determined values of model parameters at which the screening properties are preserved both in the case of a screen of limited horizontal dimensions and in the presence of a rupture in the screen comparable in horizontal size with the 3-D heterogeneity. As follows from the modeling results, the screening effect of the high resistivity model layers seriously complicates the use of MT soundings for the identification of a local heterogeneity of both a low and a high resistivity (ρ=10^?3 and 10⁴ Ω m, respectively) if its lateral size is smaller than the 4-km thickness of the studied three-layer section in question and its height is 1–1.5 km. The regular patterns of the screening effect revealed in this work are of interest in electromagnetic sounding applications.  相似文献   

Electromagnetic 3D tomography of the Elbrus volcanic center according to magnetotelluric and satellite data     

V. V. Spichak  V. P. Borisova  E. B. Fainberg  A. A. Khalezov  A. G. Goidina 《Journal of Volcanology and Seismology》2007,1(1):53-66

A geoelectric 3D model of the potentially active Elbrus volcano and its vicinities has been constructed using magnetotelluric data, which takes the volume model of the tectonic fragmentation field developed based on deciphering satellite photographs into account. An original method of searching for the correlation and determination of the character of the interrelation between the ground-based and satellite data has been used in this construction. The geoelectric 3D model constructed includes two conducting objects located at different depths. One of the objects, with a resistivity of 25–40 Ω m, located at depths of 0–10 km, is most intense at a depth of 5 km, where the object is quasi-isometric in shape and has a radius of 10 km along the 40–60 Ω m contour lines. Another object with a resistivity of 10–40 Ω m is located at a depth of ～45 km, where its dimensions along the contour line for 40 Ω m are 35 and 15 km in latitude and longitude, respectively. The thickness of the conducting core is approximately 20 km. The upper and lower objects can be interpreted as a volcano magma chamber and a volcano parent chamber, respectively.  相似文献   

Three-dimensional geoelectrical model of metallogenic zones in the Kuznetsk-Alatau folded area     

V. V. Belyavskii  A. G. Goidina 《Izvestiya Physics of the Solid Earth》2012,48(11-12):877-897

The three-dimensional (3D) geoelectric model of the Kuznetsk-Alatau folded area is reconstructed by magnetotelluric inversion using 3D fitting. It is established that the zones of ore mineralization within the Batenevsky massif are confined to the subvertical faults characterized by the electric resistivity of 100?C300 ?? m. Blocks with ?? ?? 10?100 ?? m are identified at a depth below 10 km in the western part of the model. The blocks are located close to the areas marked by the increased thermal flux, reduced seismic velocities, and elevated Moho boundary. This is probably associated with the presence of the rift zone in this area.  相似文献   

A three-dimensional crustal geoelectric model of the Ukrainian Shield     

S. N. Kulik  T. K. Burakhovich 《Izvestiya Physics of the Solid Earth》2007,43(4):284-289

A 3-D crustal geoelectric model of the Ukrainian Shield (USh) is constructed from magnetic variation data. High electrical conductivity anomalies with resistivities of 1–100 Ω m are located at depths of up to 30 km from the basement surface. A high conductivity layer with ρ = 25 Ω m and with its upper boundary at a depth of 70 km is supposed to exist in the upper mantle of the southwestern USh.  相似文献   

Comparing 2D capabilities of HEC-RAS and LISFLOOD-FP on complex topography     

Iuliia Shustikova  Alessio Domeneghetti  Jeffrey C. Neal  Paul Bates  Attilio Castellarin 《水文科学杂志》2013,58(14):1769-1782

ABSTRACT

This study evaluates and compares two-dimensional (2D) numerical models of different complexities by testing them on a floodplain inundation event that occurred on the Secchia River (Italy). We test 2D capabilities of LISFLOOD-FP and HEC-RAS (5.0.3), implemented using various grid sizes (25–100 m) based on 1-m DEM resolution. As expected, the best results were shown by the higher-resolution grids (25 m) for both models, which is justified by the complex terrain of the area. However, the coarser resolution simulations (50 and 100 m) performed virtually identically compared to the high-resolution simulations. Nevertheless, the spatial distribution of flood characteristics varies: the 50 and 100 m results of LISFLOOD-FP and HEC-RAS misestimated flood extent and water depth in selected control areas (built-up zones). We suggest that the specific terrain of the area can cause ambiguities in large-scale modelling, while providing plausible results in terms of the overall model performance.  相似文献   

3D upper crustal density structure of the Deccan Syneclise,Central India          下载免费PDF全文

K.N.D. Prasad  A.P. Singh  V.M. Tiwari 《Geophysical Prospecting》2018,66(8):1625-1640

A constrained 3D density model of the upper crust along a part of the Deccan Syneclise is carried out based on the complete Bouguer anomaly data. Spectral analysis of the complete Bouguer gravity anomaly map of the study region suggests two major sources: short wavelength anomalies (<100 km) caused primarily due to the density inhomogeneities at shallow crustal level and long wavelength anomalies (>100 km) produced due to the sources deeper than the upper crust. A residual map of the short wavelength anomalies is prepared from the complete Bouguer anomaly using Butterworth high‐pass filter (100 km cut‐off wavelength). Utilizing the constraints from deep resistivity sounding, magnetotellurics and deep seismic sounding studies, 2.5D density models have been generated along 39 profiles of this region. The mismatch between the calculated response of the a priori 2.5D model with the residual (short wavelength) gravity anomalies is minimized by introducing high‐density intrusive bodies (≥2.81 g/cm³) in the basement. With these 2.5D density models, the initial geometry of our 3D density model, which includes alluvium, Deccan trap, Mesozoic sediment and high‐density intrusive bodies in the basement up to a depth of 7 km (upper crust), is generated. In the final 3D model, Deccan trap extends from 200 m to nearly 1700 m below the 90–150 m thick Quaternary sediment. Further down, the sub‐trappean Mesozoic sediment is present at a depth range of 600–3000 m followed by the basement. The derived 3D density model also indicates six intrusive bodies of density 2.83 g/cm³ in the basement at an average depth of about 4–7 km that best fits the residual gravity anomaly of the study area.  相似文献   

A 3D resistivity model derived from the transient electromagnetic data observed on the Araba fault,Jordan     

《Journal of Applied Geophysics》2013

72 inloop transient electromagnetic soundings were carried out on two 2 km long profiles perpendicular and two 1 km and two 500 m long profiles parallel to the strike direction of the Araba fault in Jordan which is the southern part of the Dead Sea transform fault indicating the boundary between the African and Arabian continental plates. The distance between the stations was on average 50 m.The late time apparent resistivities derived from the induced voltages show clear differences between the stations located at the eastern and at the western part of the Araba fault. The fault appears as a boundary between the resistive western (ca. 100 Ωm) and the conductive eastern part (ca. 10 Ωm) of the survey area. On profiles parallel to the strike late time apparent resistivities were almost constant as well in the time dependence as in lateral extension at different stations, indicating a 2D resistivity structure of the investigated area.After having been processed, the data were interpreted by conventional 1D Occam and Marquardt inversion. The study using 2D synthetic model data showed, however, that 1D inversions of stations close to the fault resulted in fictitious layers in the subsurface thus producing large interpretation errors. Therefore, the data were interpreted by a 2D forward resistivity modeling which was then extended to a 3D resistivity model. This 3D model explains satisfactorily the time dependences of the observed transients at nearly all stations.  相似文献   

Spatial distribution of resistivity in the hydrogeological systems,and identification of the catchment area in the Rharb basin,Morocco / Répartition spatiale de la résistivité dans les systèmes hydrogéologiques et détection des zones de captages dans le bassin du Rharb,Maroc     

《水文科学杂志》2013,58(3)

Abstract

Abstract Geophysical results obtained in the Rharb basin, Morocco are reported. Correlations between hydrogeological well logs reveal several water-bearing Plio-Quaternary units resting on a substrate of blue marls. Geo-electrical borehole analyses were interpreted using bi-logarithmic diagrams which indicate the permeable layers of the aquifer and also its basement. Resistivity data from NE–SW and NW–SE electrical sections allow definition of the permeable/impermeable levels, and identification of ?ditches? that may be favourable sectors for hydrogeological exploitation. Resistivity anomalies were investigated by analysing maps of resistivity at 400 and 1000 m AB. Anomalies identified in the Rharb basin are related to the thickening of the permeable bodies (sand, limestone, sandstone deposits). In the coastal zone (AB = 1000 m), there is a strong decrease of the resistivity gradient (35–10 Ω m), which is probably linked to marine intrusion. Electrical anomalies allow detection of the water-bearing zones notably in the western and southwestern parts of the Rharb basin.  相似文献   

Bootstrap confidence intervals for design storms from exceedance series     

T. A. G. Gunasekara  C. Cunnane 《水文科学杂志》2013,58(6):719-721

Abstract

An approach for better understanding of the physical implication of estimated aquifer parameters is demonstrated by analysing the pumping test data at Cottam in the Nottingham aquifer, UK. A sensitivity analysis showed that the area represented by the estimated parameters was much smaller than the area covered by the depression cone. After parameters are estimated, further research should be carried out to understand what portions of the aquifer the parameters represent. The parameters estimated at Cottam represented mainly aquifer features between roughly 100 and 2000 m. The sensitivity analysis also showed that the observed drawdown being satisfactorily matched by a model with uniform parameters does not prove that the aquifer is homogeneous. Slightly anomalous data may imply the existence of large anomalous zones. Although the drawdowns at Cottam could be ‘satisfactorily’ fitted by a model with uniform parameters, the fit could be improved by a model using a more permeable aquifer but with a zone about 700 m wide and with 42% less transmissivity.  相似文献   

Resolution of airborne VLF data     

《Journal of Applied Geophysics》2006,58(2):158-175

The interpretation of airborne VLF data represents an important aspect of geophysical mapping of the upper few hundred meters of the Earth's crust, especially in areas with crystalline rocks. We have examined the ability of the single frequency VLF method to provide quantitative subsurface resistivity information using two generic models and standard airborne parameters with a flight altitude of 70 m and a frequency of 16 kHz. The models are long thin conductor (10 m thick, 10 Ω m resistivity and 1 km long) and a wider buried conductive dike (100 Ω m resistivity and 500 m wide). Using standard regularized inversion it turned out that for both models the conductivity of the conductors are underestimated and the vertical resolution is rather poor. The lateral positions of the minimum of the resistivity distributions coincide well with the true positions of the shallow conductors. For deeper conductors the position of the minimum resistivity moves from the edges of the conductor into the conductor. The depth to the minimum of the resistivity anomalies correlates well with the true depth to the top of the conductors although the latter is always smaller than the former.Interpretation of field airborne data collected at 70 m flight height resolved both small scale and large scale near surface conductors (conductance ∼1 S). Deeper conductors show up in the VLF data as very long wavelength anomalies that are particularly powerful in delineating the lateral boundaries of the conductors. Many of the VLF anomalies in the Stockholm area are dominated by these deep conductor responses with some near surface conductors superimposed. The deep conductors often follow topographic lows coinciding with metasediments. We interpret the frequent absence of near surface responses at 70 m flight height as a result of weak coupling between the primary VLF wave and the small scale (in all three dimensions) near-surface conductors.Radio magnetotelluric (RMT) ground measurements were carried out along a short profile coinciding with part of an airborne profile. Using data at 9 frequencies (14–250 kHz) small scale conductors in the upper few tens of meters, not identified from the airborne data, could be well resolved. Large scale deeper conductors could be identified by both methods at nearly the same positions.  相似文献   

A comparative study of results obtained in magnetotelluric deep soundings in Villarrica active volcano zone (Chile) with gravity investigations, distribution of earthquake foci, heat flow empirical relationships, isotopic geochemistry Sr/Sr and SB systematics     

M. Muoz  H. Fournier  M. Mamani  J. Febrer  E. Borzotta  A. Maidana 《Physics of the Earth and Planetary Interiors》1990,60(1-4):195-211

The first magnetotelluric deep soundings in Chile were carried out during 1986 in the Villarrica active volcano zone (39°25′S, 71°57′W). In the TM mode of polarization, the curves show a distorted segment with dispersion. A static distortion at long periods is observed in curves in the TE mode of polarization; the segment was shifted vertically to fit the geomagnetic global model values at daily periods. This modified curve was used for 1D modelling to determine the electrical structure in the study area. The upper level of the intermediate conducting layer of resistivity 20–60 ω m is found to be at 35–50 km depth. A higher resistivity layer (600 ω m), starting at 100 km depth, may be resolved in the intermediate conducting layer. A sharp decrease in the resistivity is shown by the model at 500 km.

Large heterogeneities at the level of the conducting layer encountered in the 1D modelling, and increased resistivity of the ultimate layer, may account for distortion observed at long periods. Two-dimensional test models show that the conducting layer in the area of Villarrica volcano may be an anomalous heated layer surrounded by rocks of higher resistivity of about 2 × 10³ ω m. These features correspond to the interaction with a subsiding oceanic lithosphere resulting in a complex thermal structure and perturbed resistivity distribution in transition zones of the Pacific type such as Chile, and to the existence of a megafault and a system of fractures in the sounding area. These facts make it difficult to determine the conductance of the electrical asthenosphere.

The parameters of the model structure correlate well with geophysical and geochemical results obtained in the area by other workers. Gravity studies indicate a maximum crustal thickness of about 37 km, which implies a non-full compensation according to the Airy hypothesis. The morphology of the Wadati-Benioff zone clearly shows a sharp decrease of earthquake foci at 50 km depth, and a reinitiation of seismic activity from a depth of 100 km down to 160 km. Kaufman and Keller and Levi and Lysak empirical relationships between heat flow and thickness of the lithosphere are somewhat consistent when the parameters of the model structure are assumed. The strontium isotope ratios are indicative of minimal crustal contamination of mantle-derived magmas, thus allowing a relatively thin crust under the area. Furthermore, the SB index of partial melting of mantle peridotite may suggest the contribution of uprising material from the deep asthenosphere through fractures extending to depth.  相似文献   

Very high resolution 3D marine seismic data processing for geotechnical applications     

Marsset  Missiaen  De Roeck  Noble  Versteeg  & Henriet 《Geophysical Prospecting》1998,46(2):105-120

The processing of a small-scale, very high resolution (VHR) shallow marine 3D data volume is described. The data were acquired over a small clay diapir, on the river Schelde, in 1990. Using an array of 12 dual-channel microstreamers towed from a catamaran, a network of 1 m × 1 m bins could be produced over an area of 50 m × 180 m (< 100 m penetration). Positioning was performed with an auto-tracking laser ranging system, assuring an absolute accuracy of a few decimetres.
Preliminary processing steps included tidal correction and multiple removal. An important step concerned the application of 3D prestack Kirchhoff depth migration. Indeed this processing allows easy handling of the exact positions of both source and receivers as the latter were not set out on a conventional regular grid due to navigation difficulties. Because of the restricted data volume and the more-or-less stratified medium, a 1D velocity model could be used. This allowed a considerable simplification of the migration algorithm, based on summation. Traveltimes were calculated only once, using a 2D time grid with 0.1 m intervals.
This migration method proved very efficient, greatly improving the seismic image, and involved only limited CPU time on a small computer (Sparc 10 workstation). It clearly demonstrates that advanced seismic processing can form a valuable and economically feasible tool for VHR shallow subsurface 3D seismics, as long as the velocity field is not too complex. This method should therefore no longer be restricted to large computers and hydrocarbon exploration, but should also become a routine for VHR 3D shallow seismic work.  相似文献   

Snow in Lebanon: a preliminary study of snow cover over Mount Lebanon and a simple snowmelt model / Etude préliminaire du couvert neigeux et modèle de fonte des neige pour le Mont Liban     

《水文科学杂志》2013,58(3)

Abstract

The physical properties of snow, including apparent density, snow cover distribution and snowmelt in the Nahr El Kelb basin (Mount Lebanon), were studied in order to design a simple empirical snowmelt model. In February 2001, snow covered an area of 1600 km² on Mount Lebanon, representing a water equivalent of 1.1 x 10⁹ m³. The snow surface area was calculated by combining TM5 images with a digital elevation model, and field observations made every three days, from 1400 to 2300 m altitude. The depletion of snow cover was measured from the end of December 2000 to the end of June 2001. The snowmelt was measured from surface depletion on a degree-day basis. A simple model relating the daily snowmelt to the product of wind speed and average positive daily air temperature, is presented and discussed. For Mount Lebanon, this model gave a better approximation of snowmelt than a simple degree-day model.  相似文献   

基于形变观测分析2011年日本9.0级地震与断层运动间关系          下载免费PDF全文

邵志刚  张浪平  马宏生  刘晓霞  孙小龙 《地球物理学报》2015,58(3):857-871

2011年3月11日日本发生9.0级地震,本文以此次地震的震间、同震和震后形变观测为约束,依据不同时段断层运动空间分布特征分析日本海沟地区强震与断层运动间关系.震间日本海沟地区,断层运动闭锁线深度约为60km,闭锁线以上从深到浅依次为断层运动强闭锁段、无震滑移段和弱闭锁段.由同震位错反演结果,2011年日本9.0级地震同震存在深浅两个滑移极值区,同震较浅的滑移极值区(同震位错量10~50m,深度小于30km)震间为断层弱闭锁段;同震较深的滑移极值区(同震位错量10~20m,深度在40km左右)震间为断层强闭锁段;而在两者之间的过渡带同震位错相对较小,震间断层运动表现为无震滑移.震后初期断层运动主要分布在在闭锁线以上的同震较深滑移极值区,而同震较浅的滑移极值区能量释放比较彻底,断层震后余滑量相对较小.依据本文同震和震间断层运动反演结果,震间强闭锁段积累10m同震位错需要100多年时间,与该区域历史上7级地震活动复发周期相当;震间弱闭锁段积累30~50m同震位错约需要300~600年时间,与相关研究给出的日本海沟9级左右地震复发周期比较一致.在实际孕震能力判定的工作中,由于不同性质的断层段在同震过程中会表现更多的组合形式,断层发震能力判定结果存在更多的不确定性,但利用区域形变观测等资料给出震间断层运动特征的研究工作对于断层强震发震能力的判定具有非常重要的实际意义.  相似文献   

Groundwater potential of the Egbe-Mopa basement area,central Nigeria     

C.O. Okogbue  O.V. Omonona 《水文科学杂志》2013,58(4):826-840

Abstract

An investigation on the groundwater potentials of the Egbe-Mopa area in central Nigeria, underlain by the Basement Complex, is presented. The investigation involved mapping of the subsurface by use of vertical electrical soundings; measurement of depth to groundwater; and evaluation of hydraulic conductivity, transmissivity and yield by means of pumping test interpretation. The results indicate subsurface units that range from three to five resistivity layers; depth to groundwater of 0–10 m; overburden thickness of 3–16 m; hydraulic conductivity of 6.2?×?10^?6 to 3.4?×?10^?4 m/s; transmissivity of 4.3?×?10^?7 to 2?×?10^?3 m²/s; and groundwater yield of 0.2–2.5 L/s. The hydraulic head assessments revealed a general northward groundwater flow direction. The study identified three aquifer potential types, of high, medium and low productivity, respectively. Based on the longitudinal conductance of the overburden units, four distinct Aquifer Protective Capacity zones were delineated, namely, poor, weak, moderate and good.

Citation Okogbue, C.O. and Omonona, O.V., 2013. Groundwater potential of Egbe-Mopa basement area, central Nigeria. Hydrological Sciences Journal, 58 (4), 826–840.  相似文献   

Electrical resistivity imaging and hydrochemical analysis for groundwater investigation in Kuala Langat,Malaysia     

Mahmoud Khaki  Ismail Yusoff  Nur Islami 《水文科学杂志》2013,58(4):751-762

ABSTRACT

Integrated two-dimensional electrical resistivity imaging (ERI) and hydrochemical surveys were used to investigate the groundwater alluvial aquifer in Kuala Langat, Malaysia. The study in the Langat basin considered the thickness of the aquifer, the depth of the bedrock, the regions influenced by seawater intrusion, and the monitoring of water levels. The resistivity imaging results show that the upper layer consists of clay, while the second layer is an aquifer whose thickness varies mostly in the range of 10–30 m, and in some cases extends to 40 m. The bedrock depth varies from 30 to 65 m. The chemical analyses were carried out on groundwater samples from nine boreholes collected between 2008 and 2012. The analyses indicate that the total dissolved solids (TDS) exceed 1000 mg L^-1 near the coastal area and are often less than 500 mg L^-1 further inland. The ERI and hydrochemical analyses reveal that groundwater in the study area, especially towards the coast, is a mixture of brackish and fresh waters.